1. Field of the Invention
The present invention relates to a structure of a lumbar support portion of a seat back of an automotive seat for supporting the lumbar vertebrae portion of an occupant of the automotive seat and, in particular, to a structure of a lumbar support portion of a seat back of an automotive seat which can distribute the body pressure of the occupant in a well-balanced manner.
2. Description of the Prior Art
Since the lumbar support portion of the seat back of the automotive seat must carry heavier loads given by the occupant when compared with the remaining portions of the seat back, conventionally, there have been devised various measures to cope with this problem. That is, in order to increase the supporting capability of the lumbar support portion (A), in the prior art, there have been enforced several methods: in a first method, the tensile forces of spring members (a)(b) which are provided over such lumbar support portion in a frame (4) shown in FIG. 1 are increased with respect to other spring members (c)(d)(e) provided over the remaining portions in the frame, (FIG. 2); in a second method, a projected portion (c) is provided on the front side (the lumbar vertebrae portion of the occupant) of a cushion member (1') formed of foam material, (FIG. 3); and, thirdly, a high density of foam block member (5) is embedded into the foam cushion member (1'), (FIG. 4).
However, there have been found some disadvantages in the above-mentioned methods. For example, in a structure constructed in accordance with the first method, that is constructed by increasing the tensine forces of the spring members (a)(b), as shown by a body pressure distribution diagram in FIG. 6, there is produced a steep difference (w) in body pressure responsive force between the lumbar support portion (A) and the remaining portions of the seat back. Also, in actual use, as shown in FIG. 7, if the occupant moves or rotates his or her upper body about a hip point (HP) by angle of .theta., then the pressure of the spring member (b) is increased with respect to the spring member (a) correspondingly to the occupant's movements for the angle of .theta., and thus the pressure of the spring member (b) becomes higher than a set peak line (F) (that is, an ideal design line in which a maximum loading force is applied to the lumbar vertebrae portion of the occupant in the most favorable direction thereof).
Accordingly, the conventional seat backs constructed in accordance with the above-mentioned methods cannot be tuned to its body pressure responsive force in a well-balanced manner, thus they fail to provide a comfortable sitting feeling.
In FIGS. 2 through 4, reference numeral (3) designates a trim cover assembly which forms a part of the automotive seat.